Conventional memory systems employ a single DIMM (dual inline memory module) in the space that is designated for the containment of system memory modules. In many of these systems the space designated for the containment of system memory modules have the capacity to hold additional modules. However on-board termination characteristics limit the use of this space to components that adhere to industry standards already in place.
Conventional DRAM topologies are designed to support an industry-standard DDR SDRAM DIMM design. A double-high DIMM is similar to an industry-standard DIMM (see FIG. 1 industry-standard DIMM) except that it has an extra row of DRAM chips that doubles the memory occupying the space conventionally occupied by one DIMM connector. A criterion in the design of double-high DIMMs may be that the underlying memory subsystem function with either an industry-standard DIMM, custom double-high DIMMs or both types of DIMMS. Other than the ability to mix DIMMs in the system, the double-high DIMM can be a fully custom design.
FIG. 1 is a block diagram of a conventional industry-standard DDR SDRAM DIMM circuit topology. FIG. 1 shows DIMM connector 101, resistor 103, DDR SDRAM 105, transmission line 1 and 107 transmission line 2 109. In FIG. 1 transmission line 1 107 couples DIMM connector 101 to a resistor 103 and transmission line 2 109 couples resistor 103 to DDR SDRAM 105.
DIMM connector 101 receives data input and outputs and supplies the data to internal or external circuit elements. Resistor 103 transmits data that is received at the DDR SDRAM 105 when data is to be stored and transmits data to DIMM connector 101 when data is accessed from the DDR SDRAM 105.
Industry-standard DIMMs require certain on-board termination in the memory subsystem outside the confines of the DIMM. This requirement places serious constraints on possible designs of the double-high DIMM. One way to take advantage of the additional space that is available is by employing memory unit (e.g., DRAM etc.) stacking technologies. However, the use of memory unit (e.g., DRAM etc.) stacking technologies is undesirable as the methodologies employed prevent optimal use of board space since DIMMs using memory unit stacking technologies conventionally utilize more board area than non-stacked arrangements because of thermal airflow requirements.